Cement compositions with improved mechanical properties and methods of cementing in a subterranean formation

ABSTRACT

Cement compositions having enhanced mechanical properties and comprising microspheres and carbon fibers are provided herein. The cement compositions comprise a cement material, carbon fibers, microspheres, sufficient water to form a slurry, and optionally other ingredients.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a divisional application of U.S. application Ser.No. 10/414,641 entitled “Cement Compositions with Improved MechanicalProperties and Methods of Cementing in a Subterranean Formation,” filedon Jul. 16, 2003.

BACKGROUND

The present invention relates to subterranean cementing operations, andmore particularly, to cement compositions comprising microspheres havingimproved mechanical properties, processes of preparing suchcompositions, and methods of using such compositions in subterraneancementing operations.

Hydraulic cement compositions are commonly utilized in subterraneanoperations, particularly subterranean well completion and remedialoperations. For example, hydraulic cement compositions are used inprimary cementing operations whereby pipe strings such as casings andliners are cemented in well bores. In performing primary cementing,hydraulic cement compositions are pumped into the annular space betweenthe walls of a well bore and the exterior surface of the pipe stringdisposed therein. The cement composition is permitted to set in theannular space, thereby forming an annular sheath of hardenedsubstantially impermeable cement therein that substantially supports andpositions the pipe string in the well bore and bonds the exteriorsurfaces of the pipe string to the walls of the well bore. Hydrauliccement compositions also are used in remedial cementing operations suchas plugging highly permeable zones or fractures in well bores, pluggingcracks in holes in pipe strings, and the like.

Subterranean formations transversed by well bores are often weak andextensively fractured, and may be vulnerable to the loss of circulationof cement compositions during placement. Such loss of circulation ofcement compositions during placement is of great concern, and preventionof loss of circulation often calls for reducing the density of thecement as much as possible to accordingly reduce the hydrostaticpressure. A traditional means of reducing the density of the cement hasbeen to increase the cement's water content, because, generallyspeaking, the higher the water content, the lighter the cement. However,this method may be problematic because the resultant cement often lacksthe desired strength, especially in light of the stresses to which thecement will be subjected.

Cement compositions containing lightweight spherical or substantiallyspherical compounds have been developed as an alternative means ofreducing the density of a given cement composition. Some compositionscomprise microspheres, which typically comprise cenospheres, glasshollow microspheres, or ceramic hollow microspheres. Cenospheres arehollow spheres primarily comprising silica (SiO₂) and alumina (Al₂O₃),and are filled with gas. These are a naturally occurring by-product ofthe burning process of a coal-fired power plant. The addition ofconventional glass hollow microspheres and ceramic hollow microspheresreduces the density of the cement composition and produces a lightweightcement composition, e.g., a cement composition having a density lessthan about 13 pound per gallon.

Cement failures can be particularly problematic in lightweight cementcompositions containing a high proportion of microspheres, as thesecompositions have demonstrably reduced strength. This problematicreduction in strength is believed to be attributable to the combinationof reduced content of cementitious materials and increased void space,both of which are caused by the addition of the hollow microspheres tothe cement composition. Failure of cement within the well bore canresult in radial or circumferential cracking of the cement as well as abreakdown of the bonds between the cement and the pipe or between thecement sheath and the surrounding subterranean formations. Such failurescan result in at least lost production, environmental pollution,hazardous rig operations, and/or hazardous production operations.Another undesirable result is the presence of pressure at the well headin the form of trapped gas between casing strings.

To successfully meet the subterranean challenges to which a cementcomposition may be exposed, a low-density cement composition shoulddevelop high bond strength after setting, and also have sufficientelasticity and ductility to resist loss of pipe or formation bonding,cracking and/or shattering as a result of all of the stressfulconditions that may plague the well, including impacts and/or shocksgenerated by drilling and other well operations.

SUMMARY

The present invention provides cement compositions having enhancedmechanical properties and comprising microspheres and carbon fibers, andmethods for cementing in a subterranean formation using such cementcompositions.

One embodiment of the cement compositions of the present inventioncomprise a hydraulic cement, water, microspheres and carbon fibers,wherein the carbon fibers are present in the cement composition in anamount in the range of from about 1% to about 15% by weight of cement.

The features and advantages of the present invention will be readilyapparent to those skilled in the art upon a reading of the descriptionof the embodiments that follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides cement compositions having enhancedmechanical properties and comprising microspheres and carbon fibers, andmethods for cementing in a subterranean formation using such cementcompositions. The present invention provides cement compositions havingimproved mechanical properties, including tensile strength, and methodsof utilizing these cement compositions in subterranean cementingoperations. While the compositions and methods are useful in a varietyof subterranean applications, they are particularly useful in wellcompletion and remedial operations, including primary cementing, e.g.,cementing casings and liners in well bores, including those inmulti-lateral subterranean wells.

The improved cement compositions of the present invention generallycomprise a hydraulic cement, carbon fibers, microspheres, and watersufficient to form a pumpable slurry. The cement compositions of thepresent invention generally have a density in the range of from about 5lb/gallon to about 15 lb/gallon, more preferably in the range of fromabout 6 lb/gallon to about 14 lb/gallon. Other additives suitable foruse in subterranean cementing operations also may be added to thosecompositions if desired.

Any cement suitable for use in subterranean cementing operations may beused in accordance with the present invention. In one embodiment, thecement compositions comprise a hydraulic cement. A variety of hydrauliccements are suitable for use in the compositions and methods of thepresent invention including those comprised of calcium, aluminum,silicon, oxygen, and/or sulfur, which set and harden by reaction withwater. Such hydraulic cements include but are not limited to, Portlandcements, pozzolana cements, gypsum cements, high alumina contentcements, silica cements, and high alkalinity cements.

The water utilized in the cement compositions can be fresh water, saltwater (e.g., water containing one or more salts dissolved therein),brine, or seawater. Generally, the water can be from any source providedthat it does not contain an excess of compounds that may adverselyaffect other components in the cement composition. The water may bepresent in an amount sufficient to form a pumpable slurry. Generally,the water is present in the cement compositions in an amount in therange of from about 50% to about 350% by weight of cement (“bwoc”)therein. In some embodiments, the water is present in the cementcompositions in an amount in the range of from about 60% to about 200%bwoc therein.

The microspheres that may be used in the cement compositions of thepresent invention may be any microsphere suitable for use insubterranean applications to, inter alia, reduce the density of thecement composition. As referred to herein, the term “microspheres” willbe understood to mean any body having a low specific gravity as comparedwith conventional filler, such that when used as a filler in a cementcomposition, they contribute to reducing the weight of the cementcomposition. In some embodiments of the cement compositions of thepresent invention, the microspheres utilized are synthetic, non-porousmicrospheres. The microspheres may be obtained from any suitable source.Particularly suitable microspheres are cenospheres, which are hollowmicrospheres that are formed as an industrial waste by-product, andwhich are commercially available from, for example, Halliburton EnergyServices, Inc., of Duncan, Okla., under the tradename “SPHERELITE.”Generally speaking, the microspheres are present in the cementcompositions in the amount of from about 10% to about 390% bwoc. In someembodiments, the microspheres are present in an amount in the range offrom about 20% to about 100% bwoc. The microspheres may be added to acement composition by any suitable method including by dry blending withthe cement before the addition of a fluid such as water, by mixing withthe fluid to be added to the cement, or by mixing with the cement slurryconsecutively with or after the addition of the fluid. The microspheresmay be pre-suspended in water and injected into the cement mix fluid orinto the cement slurry as an aqueous slurry. Preferably, themicrospheres are dry blended with the cement before the addition ofwater.

It has been found that adding microspheres to a cement compositionaffects the mechanical properties of the cement composition, inter alia,by lowering the cement composition's density. However, when microspheresare added in quantities sufficient to desirably affect the density ofthe cement, the mechanical properties of the cement are also affected,e.g., its tensile strength. The reduction in tensile strength can beparticularly problematic because the risk of rupture of the cementsheath in response to a stressful condition is directly linked to thetensile strength of the cement. Moreover, the risk is attenuated whenthe ratio of the tensile strength of the cement to its Young's Modulusis increased. Thus, adding carbon fibers to a cement composition thatcomprises microspheres is desirable to enhance the mechanical propertiesincluding the tensile strength of such cement compositions. Also, addingcarbon fibers as opposed to other additives, such as polypropylene, hasthe added benefit of providing increased temperature stability to thecement composition. This makes the cement compositions of the presentinvention especially suitable for use in or in conjunction with hostilewell bore conditions, such as high temperatures and/or high pressures.

The carbon fibers that are present in the cement compositions of thepresent invention preferably have a relatively high tensile strengthand/or a relatively high tensile modulus. In certain embodiments, toachieve certain specific/desired advantages associated with the presentinvention, the tensile modulus of the fibers may exceed 180 GPa, and thetensile strength of the fibers may exceed 3000 MPa. The fiberspreferably have a mean length of about 1 mm or less. In certainembodiments, the mean length of the carbon fibers is from about 50 toabout 500 microns. Most preferably, the fibers have a mean length in therange of about 100 to about 200 microns. Preferably, they are milledcarbon fibers. An example of suitable carbon fibers includes “AGM-94”carbon fibers commercially available from Asbury Graphite Mills, Inc.,of Asbury, N.J. AGM-94 fibers have a mean length of about 150 micronsand a diameter of about 7.2 microns. Another example of suitable carbonfibers includes the “AGM-99” carbon fibers, also available from AsburyGraphite Mills, Inc., which have a mean length of about 150 microns anda diameter of about 7.4 microns. In some embodiments, the carbon fibersare present in the cement compositions in an amount in the range of fromabout 1% to about 15% bwoc.

Additional additives may be added to the cement composition as deemedappropriate by one skilled in the art for improving or changing theproperties of the ensuing hardened cement. Examples of such additivesinclude but are not limited to, inter alia, fluid loss controladditives, defoamers, surfactants, vitrified shale, mica, fiber, flyash, bentonite, fumed silica, salts, dispersing agents, setaccelerators, formation conditioning agents, and set retarders. Anexample of a suitable dispersing agent is a water-soluble polymerprepared by the caustic catalyzed condensation of formaldehyde withacetone wherein the polymer contains sodium sulfate groups. Suchdispersing agent is commercially available under the trade designation“CFR-3” from Halliburton Energy Services of Duncan, Okla. Anothersuitable dispersant is commercially available under the tradedesignation “CFR-2” from Halliburton Energy Services in Duncan, Okla.

An example of a preferred cement composition of the present inventioncomprises: Class G Portland cement; 98% sea water bwoc; 1% CFR-3dispersant bwoc; 55% hollow microspheres bwoc; and 10% milled carbonfibers bwoc having a mean length of 150 microns.

To facilitate a better understanding of the present invention, thefollowing examples of specific embodiments are given. In no way shouldthe following examples be read to limit, or define, the entire scope ofthe invention.

EXAMPLE 1

Test samples of preferred exemplary embodiments of the cementcompositions of the present invention were made and the tensile strengthof each composition was determined. Comparative samples were also madeand similarly tested. To prepare the samples of the cement compositions,Class G Portland cement was mixed with 98% sea water bwoc and 55%SPHERELITE bwoc, and cured for 72 hours at 150° F. To certain samplecement compositions, 1% CFR-3 dispersant bwoc and carbon fibers wereadded in chosen ratios as described in Table 1. The carbon fibers weremilled fibers, specifically AGM-94 fibers from Asbury Graphite MillsInc., with a mean length of 150 microns and a diameter of 7.2 microns.The tensile strength of each cement composition was then determined. Alltests were performed in accordance with ASTM C496-96.

Table 1 below lists the percentage of carbon fibers that were added toeach cement composition and the resultant tensile strength. TABLE 1Sample Dispersant Milled Carbon Fibers Tensile Description (bwoc) (bwoc)Strength (psi) Comparative None 0 183 Sample No. 1 Sample No. 2 1% CFR-3 10% 283

Comparative Sample No. 1 illustrates the tensile strength of a cementcomposition comprising microspheres when no carbon fibers have beenadded to the composition. The tensile strength was 183 psi.

Sample No. 2 illustrates the tensile strength of a cement composition ofthe present invention, comprising microspheres to which carbon fibersand a dispersant have been added. The tensile strength was 283 psi, a55% increase from Comparative Sample No. 1.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Whilenumerous changes may be made by those skilled in the art, such changesare encompassed within the spirit of this invention as defined by theappended claims.

1. A cement composition comprising: hydraulic cement; water;microspheres; and carbon fibers present in the cement composition in anamount in the range of from about 1% to about 15% by weight of cement.2. The cement composition of claim 1 wherein the hydraulic cementcomprises at least one of the following: a Portland cement; a pozzolanacement; a gypsum cement; a high alumina content cements; a silicacements; a high alkalinity cements; and a calcium phosphate cement. 3.The cement composition of claim 1 wherein the carbon fibers have a meanlength of about 150 microns.
 4. The cement composition of claim 1wherein the carbon fibers have a mean length of about 1 mm or less. 5.The cement composition of claim 1 wherein the carbon fibers have a meanlength of about 50 to about 500 microns.
 6. The cement composition ofclaim 1 wherein the carbon fibers have a mean length of about 100 toabout 200 microns.
 7. The cement composition of claim 1 wherein thecarbon fibers are milled.
 8. The cement composition of claim 1 whereinthe carbon fibers have a tensile modulus greater than about 180 GPa. 9.The cement composition of claim 1 wherein the carbon fibers have atensile strength greater than about 3000 MPa.
 10. The cement compositionof claim 1 wherein the cement composition has a tensile strength greaterthan about 183 psi.
 11. The cement composition of claim 1 wherein themicrospheres are present in the cement composition in an amount in therange of from about 10% to about 390% by weight of cement.
 12. Thecement composition of claim 1 wherein the microspheres are present inthe cement composition in an amount in the range of from about 20% toabout 100% by weight of the cement.
 13. The cement composition of claim1 wherein the water is present in the cement composition in an amount inthe range of from about 50% to about 350% by weight of cement.
 14. Thecement composition of claim 1 wherein the density of the cementcomposition is in the range of from about 5 pounds per gallon to about15 pounds per gallon.
 15. The cement composition of claim 1 wherein thedensity of the cement composition is in the range of from about 6 poundsper gallon to about 14 pounds per gallon, wherein the carbon fibers havea mean length of about 100 to about 200 microns, and wherein themicrospheres are present in an amount in the range of from about 20% toabout 100% by weight of cement.
 16. The cement composition of claim 1wherein the microspheres are pre-suspended in water before beingintroduced into the composition.
 17. The cement composition of claim 1wherein the microspheres are dry blended with the cement before theaddition of water.
 18. The cement composition of claim 1 furthercomprising at least one of the following: a fluid loss control additive,a defoamer, a surfactant, vitrified shale, mica, fiber, fly ash,bentonite, fumed silica, a salt, a dispersing agent, a set accelerator,a formation conditioning agent, and a set retarder.
 19. A cementcomposition comprising: hydraulic cement; water; microspheres present inthe cement composition in an amount in the range of from about 10% toabout 390% by weight of cement; and carbon fibers present in the cementcomposition in an amount in the range of from about 1% to about 15% byweight of cement.
 20. A cement composition comprising: hydraulic cement;water; microspheres present in the cement composition in an amount inthe range of from about 10% to about 390% by weight of cement; andcarbon fibers having a mean length of about 1 mm or less present in thecement composition in an amount in the range of from about 1% to about15% by weight of cement.